Regulation of the acidity (pH) of internal fluids within a narrow range is essential for all animals. The taste system plays a role in guarding against excessive intake of acid in foods and beverages with hyperacidity evoking an intense sour taste than is innately aversive. The mechanisms by which taste receptor cells respond to acid (sour taste transduction) will be studied using taste buds and taste cells isolated from rodents. Preliminary experiments indicate that changes in extracellular pH (acid stimulation) are mirrored by proportionate changes in intracellular pH, suggesting that pH-sensing is an intracellular process. In addition, changes in intracellular calcium levels alter internal pH, indicating that calcium may be involved in the acid-sensing transduction pathway. This hypothesis will be tested using pH- and calcium-sensitive fluorescent dyes to follow acid induced changes in intracellular pH and possible subsequent changes in intracellular calcium. Changes in membrane conductance during acid stimulation will be assessed using patch-clamp recording from isolated taste cells to help establish the nature of the predicted proton transport pathway. Complimentary experiments will involve a preparation which permits recording from chorda tympani taste nerve fibers while voltage-clamping the receptor epithelium. This powerful technique will be useful in determining whether the acid-sensing portions of the cells are apical or basolateral. Taste responses to ammonium salts share many of the properties of the acid sensing system and will be examined in a separate series of experiments using similar techniques. The results of these studies will provide new insights into sour (acid) taste reception, which plays an important role in pH balance.